This invention relates to digital switching systems in general and, more particularly, to an improved arrangement for providing a novel dc-to-ac converter as modified to provide an improved audio amplifier. In one aspect, the invention is concerned with first and second switching means which are respectively associated with the primary and secondary sides of an isolation transformer, the first switching means being driven by a square wave reference signal of fixed phase and the second switching means being driven by the square wave reference signal but in phase modulated relation to this reference signal such that the logic combination of the reference signal and the phase shifted signal provide a pulse width modulated output signal under the control of input means for providing a variable signal which is to be amplified. The variable signal is the audio signal input.
A principal object of the present invention is to provide a digital audio amplifier which, in comparison with the prior art, has a lower component count, simpler manufacture, and further reduced weight and size, yet retains the performance required of a high-fidelity audio system.
The present invention is based on the use of phase modulated PWM rather than simple PWM.
For high-fidelity audio output, which is the principal field of the present invention, it is necessary to have low distortion, less than 0.25% THD and preferably less than 0.1% THD, and to achieve this the system should be capable of switching many times, preferably at least ten times, the highest audio frequency. It is also desirable that the amplifier be capable of stereo operation without an additional isolation transformer.
In one aspect of the invention, a simplified exemplary schematic block digital controlled switching circuit comprises a dc power source, first switching means connected to said power source and operable to provide a square wave output, second switching means connected to receive said square wave output, phase control means connected to control the switching of said second switching means at a phase angle relative to the square wave output which is a function of a control signal, and diode means for controlling the polarities of the outputs of said second switching means to provide a phase modulated pulse width modulation thereof.
In another aspect, the invention involves a digital controlled amplifier circuit, including a dc power source, first oscillator means for converting said dc power source to a reference signal which is a square wave, second oscillator means for producing a sawtooth signal which is synchronized with but at twice the frequency of said reference signal, input means for providing a variable signal which is to be amplified, comparator means receiving said variable signal and said sawtooth signal for producing a phase shifted square wave signal which is said reference signal shifted in phase in accord with the instantaneous amplitude of said variable signal, logic means receiving said reference signal and said phase shifted signal for producing a pulse width modulated output signal derived from the logic combination of said reference signal and said phase shifted signal, output filter means receiving said output signal for providing an amplified output of said means for providing a variable signal, and diode means for controlling the phase modulation of said pulse width modulated output signal.